An Efficient way of generating a legend for 'for loops plots'

Dear all,

I wrote a matlab code to plot various data from a FEM analysis. My problem is creating a legend efficiently. The code uses for loops to plot an different objects multiple times. If i do not specify the objects clearly, the legend function will see thousands of different graphic objects. For this reason my code does something similar to this:

for i = 1 : n
    h = plot3(variable(i, 1), variable(i, 2), variable(i, 3));
end
legend(h, 'myvariable')

(the only difference is that h actually stores the information from different lines/data, it is a vector of handles).

The thing i do not like is that i am saving the information about my graphic object multiple time. I have not tested this but i think is clear it will increase the overall plotting time (the amount of data i am plotting can be really large). My solution to this was the following:

h = 0;
for i = 1 : n
    if h == 0
        h = plot3(variable(i, 1), variable(i, 2), variable(i, 3));
    else
        plot3(variable(i, 1), variable(i, 2), variable(i, 3));
    end
end
legend(h, 'myvariable')

This is likely to work but it will double the length of my code that right now is already 540 lines. Honestly i prefer a compact code, hence my questions: is there any other solution apart from the one I proposed? does anybody have any idea on how i can solve this problem? Am i doing it right?

Thank you!

(PS note: the for loops are necessary, i need to plot line segments which are not connected to each other)

[Rafael Hernandez-Walls]

if h== 0

[Lorenzo Chiaverini]

yes sorry, let me correct that

[Rik]

Why do you want to avoid the first version? Are you afraid that storing the handles will slow your code down?

[Lorenzo Chiaverini]

yes, (it is already pretty slow and i was trying to optimise it)

[Jan de Jong]

One way of avoiding a multitude of line segments you could store the variables into a single vector interleaved with nans.

var1 = randn(6,3);var2 = randn(6,3);
mastervar = [var1;nan(1,3);var2]; 
plot3(mastervar(:,1),mastervar(:,2),mastervar(:,3))

I do not think storing handle h to your plot will increase the computational burden. It is just a link.

Answer 1

[Steven Lord]

Two suggestions:

1) When you create the graphics object, set its DisplayName property. Then when you want to show the legend, you won't have to specify the names.

x = 0:360;
axis([0 360 -1 1])
hold on

h = gobjects(1, 2);
h(1) = plot(x, sind(x), 'DisplayName', 'sine');
h(2) = plot(x, cosd(x), 'DisplayName', 'cosine');
legend
% or to show just one
legend(h(2))

2) If the lines that you're drawing are all part of the same "thing", rather than creating a lot of small (2 point) line objects you can include NaN values in your data to be plotted to break the line at that point.

figure
x = 1:15;
x(mod(x, 3) == 0) = NaN;
h = plot(x, x)

h = 
  Line with properties:

              Color: [0 0.4470 0.7410]
          LineStyle: '-'
          LineWidth: 0.5000
             Marker: 'none'
         MarkerSize: 6
    MarkerFaceColor: 'none'
              XData: [1 2 NaN 4 5 NaN 7 8 NaN 10 11 NaN 13 14 NaN]
              YData: [1 2 NaN 4 5 NaN 7 8 NaN 10 11 NaN 13 14 NaN]
              ZData: [1×0 double]

  Show all properties

5 line segments visually, but just one graphics object in memory.

[Lorenzo Chiaverini]

Thank you for your answer, both suggestions will definitely improve my code. I ask you another question: can you do the NaN trick also with the fill3 and quiver3 commands?

Also I want to be sure the performance will be improved if i add the NaNs since I need to do many modifications to implement it.

Thank you again.

[Steven Lord]

I'm not certain about fill3. quiver3 can accept arrays of coordinates so you could potentially create all the arrows at once. If you're adding arrows in a loop I would consider creating one quiver3 object first and adding to the various *Data properties in the loop.

I can't guarantee performance will be improved. I think it very likely since you're going to reduce the number of objects and therefore the duplicate copies of the properties that are common to all plot, quiver, etc. objects.

[Lorenzo Chiaverini]

I see, that is great. Yes i am adding arrows in a loop, i will try to preallocate the object as you said and see what happens. Thank you!

[Lorenzo Chiaverini]

Just to let everybody interested know, now the plotting time is a way faster, from 200s to about 10. Thank you again.